1. Field of the Invention
The present invention relates to an optical system, which is suitable for optical apparatuses such as cameras for silver-haloid films, digital still cameras, video cameras, telescopes, binoculars, and projectors.
2. Description of the Related Art
In general, in optical systems used for image-taking apparatuses such as digital cameras and video cameras, the optical performance thereof tends to be decreased by increase of various aberrations, especially longitudinal chromatic aberration, chromatic aberration of magnification and the like, with shortening the total lens length (total optical length; the length from a first lens surface on an object side to an image plane) for attempting the miniaturization of the entire optical system. Especially, in an optical system of a telephoto type whose lens length is shortened, lengthening its focal length increases chromatic aberrations.
As methods for reducing such chromatic aberrations, the use of an anomalous partial dispersion material as an optical material and the use of a diffraction optical element in an optical path are well known.
In the telephoto type optical systems, generally, a lens unit that is located on the front side of a pupil, where passing heights of a paraxial marginal ray and a paraxial chief ray from the optical axis become relatively large, is constituted by a lens having a positive refractive power, which is formed of a low dispersion optical material with anomalous partial dispersion such as fluorite, and a lens having a negative refractive power, which is formed of a high dispersion optical material to reduce the chromatic aberrations. Such various telephoto type optical systems have been proposed (see Patent Documents 1 to 3).
Moreover, an example that the chromatic aberrations of the telephoto type optical system were corrected by using a diffraction optical element, without using the anomalous partial dispersion material, is also known (see Patent Documents 4 and 5).
Generally, the absolute value of a numerical value corresponding to Abbe's number of a diffraction optical element is a small number of 3.45. Therefore, the diffraction optical element is characterized in that the slight change of its power (a power is the reciprocal of its focal length) by diffraction causes the great change of the chromatic aberrations with little impact on spherical aberration, coma aberration, astigmatism or the like. Moreover, the power changes linearly for the change of wavelength of incident light because the light treated is diffraction light. Thereby, the wavelength characteristic of chromatic aberration coefficients forms a complete straight line.
Therefore, aberration corrections specialized for the spherical aberration, coma aberration, and astigmatism are chiefly required when shortening the total lens length. On the other hand, since the correction of the chromatic aberration of magnification is performed by using a diffraction optical element, the optical system can be designed only by optimizing grass materials and power of constituent lenses so that linearity of the wavelength characteristic of chromatic aberration coefficients may be obtained, without a consideration for the absolute amount of the chromatic aberration deteriorating due to the total length shortening. As a result, an optical system of the telephoto type whose total lens length is shortened can be obtained.
Moreover, a liquid material, with comparatively high dispersion and a property like anomalous partial dispersion, is known as an optical material having a chromatic aberration correction action that is similar to a diffraction optical element, an optical system with the liquid material having been proposed (see Patent Document 6).
[Patent Document 1]
    Japanese Examined patent application No.S60-49883 (corresponding to U.S. Pat. No. 4,241,983)[Patent Document 2]    Japanese Examined patent application No.S60-55805 (corresponding to U.S. Pat. No. 4,348,084)[Patent Document 3]    Japanese Patent Laid-Open Application No.H11-119092 (corresponding to U.S. Pat. No. 6,115,188)[Patent Document 4]    Japanese Patent Laid-Open Application No.H6-324262 (corresponding to U.S. Pat. No. 5,790,321)[Patent Document 5]    Japanese Patent Laid-Open Application No.H6-331887 (corresponding to U.S. Pat. No. 5,629,799)[Patent Document 6]U.S. Pat. No. 4,913,535
In the telephoto type optical systems, which use fluorite or the like as optical materials, disclosed in Patent Documents 1 to 3, when the total lens length is set comparatively long, the correction of the chromatic aberration is easy. However, shortening the total lens length causes the increase of the chromatic aberration, and thereby the excellent correction thereof becomes difficult. This is because, in these optical systems, the chromatic aberration generating in a front lens system with a positive refractive power is merely reduced by the use of low dispersion and anomalous partial dispersion of the material such as fluorite.
When the chromatic aberration that was deteriorated by shortening the total lens length is corrected by using a low dispersion grass with a large Abbe's number such as fluorite, since the chromatic aberration does not change greatly without the great change of the refractive power of the lens surface is not greatly changed, it becomes difficult to correct both the chromatic aberration and various aberrations such as spherical aberration, coma aberration and astigmatism that are generated by enlarging the refractive power.
On the other hand, though the diffraction optical element has an enough correction action for the chromatic aberration, it generates diffraction light of unnecessary diffraction order other than diffraction light of a designed diffraction order that is actually used. The unnecessary diffraction light becomes colored flare light deteriorating the image formation performance of the optical system.
There is a method for reducing the unnecessary diffraction light, which uses a so-called laminated diffraction optical element that a plurality of blaze type diffraction gratings are laminated in the optical axis direction, the laminated diffraction optical element concentrating energy into the designed diffraction order to reduce the unnecessary diffraction light. However, the problem that flares produced by the unnecessary diffraction light appears when taking images of high luminance objects, still remains.
Moreover, a production method of diffraction optical elements, which molds a resin material such as UV-curable resin, is known. However, since this method requires a very high molding accuracy because the diffraction efficiency of the diffraction optical element is sensitive to manufacturing error, it causes a high manufacturing cost.
Since the material disclosed in Patent Document 6 is a liquid material, the structure to seal it is needed. Therefore, the use of the material makes the manufacture of the optical system difficult. Moreover, since there is a problem of the change of characteristics such as a refractive index and dispersion by temperature, too, it cannot be said that the environment resistance of the material is enough. In addition, there is a fault that the sufficient correction action of the chromatic aberration is hard to obtain because an interface with air cannot be formed, in addition to a comparatively large Abbe's number and comparatively small anomalous partial dispersion.